refactor(release): centralize package location resolution

[jasonkuhrt]

Closes #150

Summary

• centralize repo-relative package path resolution in a single PackageLocation abstraction
• use that abstraction for configured package fallback, forecast source URLs, and PR diff package matching
• thread git root through recon and add focused custom-layout coverage plus rationale notes

Verification

• bun run --cwd packages/release check:types
• bun run --cwd packages/release test packages/release/src/api/analyzer/package-location.test.ts packages/release/src/api/analyzer/workspace.test.ts packages/release/src/api/forecaster/forecast.test.ts packages/release/src/cli/pr-preview-diff.test.ts packages/release/src/api/explorer/explore.test.ts
• bun run release:verify

Review

• subagent review pass completed with no findings

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This change is